For example, for a belt-type continuously variable transmission etc., with pulleys having a varying belt winding diameter (V-groove width), a transmission belt is used that has (a) an endless annular hoop formed by overlapping a plurality of thin strip-shaped rings and (b) a multiplicity of elements supported by the hoop in an annularly arranged state along the hoop, the elements including a saddle surface pressed against an inner circumferential surface of a first ring located at the innermost circumference of the hoop, and (c) the transmission belt is wound between a plurality of pulleys to transmit power and has a shape in the width direction of the first ring elastically deformed in accordance with the saddle surface in a pressed state while the inner circumferential surface of the first ring is pressed against the saddle surface. A transmission belt described in Patent Document 1 is an example thereof and the saddle surface forms a curved shape with a center portion in the width direction protruded outward, and the first ring is elastically deformed in accordance with the curved shape of the saddle surface in the pressed state such that the inner circumferential surface thereof is brought into close contact with the saddle surface across substantially the entire area in the width direction.
In this description, the width direction of the saddle surface is defined as the direction same as the width direction of the first ring of a thin strip shape.
FIG. 6 is a diagram for explaining an example of such a conventional transmission belt, i.e., a schematic of an engagement state between a first ring 102 located at the innermost circumference of a hoop 100 and a saddle surface 106 of an element 104, depicting a cross-sectional shape in the width direction of the first ring 102 of a thin strip shape. The saddle surface 106 forms a curved shape with a center portion gently bulging out into a protruded shape toward the outside, i.e., the outer circumferential side (upside of FIG. 6) with a curvature radius Rs around a curvature center Ps on a center line S defined at substantially the center in the width direction. Although the first ring 102 forms a curved shape with a center portion gently bulging out into a protruded shape toward the outside with a curvature radius Rr greater than the curvature radius Rs as indicated by a dashed-dotted line in a natural state, a portion wound around a pulley is pressed against the saddle surface 106 due to tension and is therefore elastically deformed in accordance with the curved shape of the saddle surface 106 as indicated by a solid line and the inner circumferential surface is brought into close contact with the saddle surface 106 across substantially the entire area in the width direction. The first ring 102 is automatically adjusted to a position in the width direction such that a curvature center Pr is substantially positioned on the center line S of the saddle surface 106. Since FIG. 6 wholly depicts a cross-section, hatching (diagonal lines) indicative of a cross-section is not depicted.